1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device which is called a lateral electric field type.
2. Description of the Related Art
A liquid crystal display device which is called a lateral electric field type has a construction in which a pixel electrode and a counter electrode which causes an electric field (a lateral electric field) having a component parallel to transparent substrates to be generated between the counter electrode and the pixel electrode are formed in each liquid crystal-side pixel area of one of the transparent substrates disposed to oppose each other with a liquid crystal interposed therebetween.
The lateral electric field type of liquid crystal display device is constructed so that the amount of light to be transmitted through the area between the pixel electrode and the counter electrode is controlled by the driving of the liquid crystal to which the electric field is applied.
Such a liquid crystal display device is known as a type which is superior in so-called wide viewing angle characteristics which enable a displayed image to be unchanged even when its display surface is observed from an oblique direction.
In this kind of liquid crystal display device, the pixel electrode and the counter electrode have so far been formed of a conductive layer which does not transmit light therethrough.
In recent years, a liquid crystal display device constructed in the following manner has been known: a counter electrode made of a transparent electrode is formed over the entire area of a pixel area except the periphery thereof, and strip-shaped pixel electrodes are formed on the counter electrode with an insulating film interposed therebetween, in such a manner as to be extended in one direction (a first direction) and to be juxtaposed in a direction (a second direction) intersecting with the one direction.
The liquid crystal display device having this construction causes a lateral electric field to be generated between each of the pixel electrodes and the counter electrode, and is still superior in wide viewing angle characteristics and is greatly improved in aperture ratio.
Incidentally, this art is described, for example, in SID 99 DIGEST: pp. 202-205.
However, the structure of the liquid crystal display device having this construction is such that, as described above, the pixel electrode and the counter electrode which causes an electric field to be generated between the pixel electrode and the counter electrode are formed as different layers with the liquid crystal interposed therebetween, so that the electric field is distributed in a larger amount in a portion away from the liquid crystal than on the side of the liquid crystal.
It has been pointed out, therefore, that in order to generate an electric field fully distributed in the liquid crystal, it is necessary to apply a comparatively large voltage (drive voltage) between the pixel electrode and the counter electrode.
In addition, it has been pointed out that since the counter electrode and the pixel electrode are superposed on each other over a large area with the insulating film interposed therebetween, a load increases during the driving of a thin film transistor (switching element) interposed between a video signal line and the pixel electrode.
The invention has been made on the basis of the above-described circumstances, and aims to provide a liquid crystal display device in which its drive voltage can be reduced.
The invention also aims to provide a liquid crystal display device in which a load (load capacitance) can be reduced during the driving of a thin film transistor.
A representative aspect of the invention disclosed in the present application will be described below in brief.
A liquid crystal display device according to the invention includes a pixel electrode and a counter electrode which are respectively disposed in different layers in a liquid crystal-side pixel area of one of transparent substrates disposed to oppose each other with a liquid crystal interposed therebetween. The counter electrode causes an electric field having a component parallel to the transparent substrates to be generated between the counter electrode and the pixel electrode. One of the pixel electrode and the counter electrode is formed as a layer closer to the liquid crystal than the other electrode is to the liquid crystal, and the other electrode, is formed as a transparent electrode extended outwardly from at least a periphery of an area on which the one electrode is superposed, and a conductive layer is formed which is capacitive-coupled between the pixel electrode and the counter electrode.
In the liquid crystal display device constructed in this manner, since the conductive layer is capacitive-coupled between the pixel electrode and the counter electrode, an electric field can be generated between the one electrode and the other electrode, and this electric field has a component parallel to the transparent substrates.
Moreover, the potentials of the capacitive-coupled electrodes are close to each other, and the capacitance itself between the electrodes lowers.
Accordingly, part of the electric field occurring between the one electrode and the other electrode is distributed to an electric field occurring between the one electrode and the conductive layer, whereby the distribution of the entire electric field can be shifted to a side closer to the liquid crystal.
This fact makes it possible to improve the efficiency of the electric field required for controlling the optical transmissivity of the liquid crystal in the case of a constant drive voltage. From another point of view, it is possible to achieve the advantage of reducing the drive voltage in the case where the electric field suffices to control the optical transmissivity of the liquid crystal.
In addition, the potential difference between the conductive layer and the other electrode which are capacitive-coupled to each other is decreased by a divided capacitance potential value, whereby it is possible to achieve the advantage that the load capacitance of the thin film transistor can be decreased.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.